Water is vital to all living plants and animals, including humans. For human consumption, water treatment is often required because natural water is usually contaminated with a large number of dissolved, colloidal and suspended materials that may cause undesirable health effects. Some of them are relatively easy to be removed, while the others are more difficult. For example, suspended materials, such as silt, fine sand, and decaying biological solid matter, can be readily removed by physical separation processes such as filters and clarifiers. Dissolved materials are harder to be removed; their removal generally needs chemical precipitation, adsorption, ion-exchange, chemical oxidation, biological oxidation and assimilation, and membrane desalination. These processes tend to be very expensive.
Despite its high cost, removal of dissolved materials is technologically feasible. However, many of the removal technologies generate a large volume of waste streams such as sludge and brine, which represents a loss of valuable water. In addition, these waste streams require further treatment and disposal. For example, current reverse osmosis (RO) technology, which is a type of membrane desalination process, removes more than 99% of dissolved materials from contaminated water and produce purified water at a recovery rate of up to 80%, while producing a concentrate stream called brine.
One of the challenges facing membrane desalination is that dissolved and colloidal silica (silicon dioxide, SiO2), which precipitates on the membrane surface, fouls the membrane, increases pressure across the membrane, and reduces the flow rate and water recovery. For example, at a recovery rate of 80%, concentrations of dissolved constituents in the brine stream from RO may be up to five times higher than those in the feed water. In this scenario, precipitation of silica may occur when silica concentration in the feed water is higher than 20 mg/L because silica has a solubility limit of about 100 mg/L.
Precipitation and solubilization of silica depend on a number of factors, including pH, temperature, and presence of other dissolved substances. Typical silica concentrations in natural water range from 5 to 25 mg/L, although concentrations near its saturation (>100 mg/L) occur in some areas (ASTM, 2010). Groundwater, urban and agricultural runoff, and wastewater usually contain more silica than surface water. Wells in volcanic and oil fields may contain up to 300 ppm of dissolved silica (Ning 2002). There are a number of processes that can remove silica from water. For example, aluminum sulfate (alum) can coagulate colloidal silica and anion exchange can remove anionic silicates. Activated alumina can also remove silica via adsorption. However, these processes are often ineffective or incomplete due to silica's complex chemistry and strict pH control, as well as the presence of interferences, such as competing anions including sulfate (SO42−), bicarbonate (HCO3−) and chloride (Cl−). In addition, these processes generate waste by-products, such as spent media, brine, and sludge, requiring proper disposal.
Thus, a need exists to develop a method and/or system to reduce the amount of silica and other contaminants in the source water prior to a desalination process to improve the efficiency thereof and to prolong the functional life of the filter used therein.